wooshoofoo, the reason mimec is calling mt_rand multiple times is because the largest number mt_rand can produce is 2^31 (2147483647, as reported by mt_getrandmax() on my server).  RFC 4122 requires a 128 bit value.

Also they are not "4 digit sequeces", but 4 digit hexadecimal numbers.  16^4 == 2^16.

mimec's limiting each random result to 2^16 avoids problem of PHP's 2^32 integer max (http://php.net/manual/en/language.types.integer.php).

If you want to call mt_rand fewer times:  mimec's version calls mt_rand 8 times ( 16 bits * 8 = 128 bits ).  You *could* call mt_rand 5 times ( 31 bits + 31 bits + 31 bits + 31 bits + 4 bits = 128 bits ).  But then you would have keep all your values as strings.

Something like: 

<?php
/**
 * Another (ugly) "random or pseudo-random" version of RFC 4122
 *
 * This version calls mt_rand() the fewest possible times.
 * if mt_getrandmax() == 2^31 then this will call mt_rand() 5 times YMMV
 *
 * Personally, I would use mimec's version
 * To handle the large values, we'll keep everything as strings.
 *
 * @return string
 */
function uuid() {    

     // Generate 128 bit random sequence 
     $randmax_bits = strlen(base_convert(mt_getrandmax(), 10, 2));  // how many bits is mt_getrandmax()
     $x = '';
     while (strlen($x) < 128) {
         $maxbits = (128 - strlen($x) < $randmax_bits) ? 128 - strlen($x) :  $randmax_bits;
         $x .= str_pad(base_convert(mt_rand(0, pow(2,$maxbits)), 10, 2), $maxbits, "0", STR_PAD_LEFT);
     }

     // break into fields
     $a = array();
     $a['time_low_part'] = substr($x, 0, 32);
     $a['time_mid'] = substr($x, 32, 16);
     $a['time_hi_and_version'] = substr($x, 48, 16);
     $a['clock_seq'] = substr($x, 64, 16);
     $a['node_part'] =  substr($x, 80, 48);
     
     // Apply bit masks for "random or pseudo-random" version per RFC
     $a['time_hi_and_version'] = substr_replace($a['time_hi_and_version'], '0100', 0, 4);
     $a['clock_seq'] = substr_replace($a['clock_seq'], '10', 0, 2);

    // Format output
    return sprintf('%s-%s-%s-%s-%s',
        str_pad(base_convert($a['time_low_part'], 2, 16), 8, "0", STR_PAD_LEFT),
        str_pad(base_convert($a['time_mid'], 2, 16), 4, "0", STR_PAD_LEFT),
        str_pad(base_convert($a['time_hi_and_version'], 2, 16), 4, "0", STR_PAD_LEFT),
        str_pad(base_convert($a['clock_seq'], 2, 16), 4, "0", STR_PAD_LEFT),
        str_pad(base_convert($a['node_part'], 2, 16), 12, "0", STR_PAD_LEFT));
}

?>

However, I think mimec's version is much more elegant.